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Fixed Oils and Skin Penetration - Temperature and Viscosity
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There are other influencing factors, for example viscosity of the penetrant; viscosity is the 'stickiness' or otherwise affecting the flow of a substance - water has a low viscosity and treacle has a high viscosity . Viscosity plays a part with some carrier oils, some being relatively viscose ( almond, olive) and which are absorbed very slowly; it is known that high viscosity fats such as lard and wool fat retard or prevent percutaneous absorption (Macht 1983).
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Other oils which are less viscous (grapeseed,linseed) are relatively quickly absorbed as tests involving the abdominal skin of the rat have shown. However, avocado oil has the reputation of being easily absorbed into the skin and of aiding the penetration os substances dissolved in it, despite its relatively high viscosity.Temperature is also a consideration as any oil becomes less viscous with a rise in temperature: therefore it can be expected that warming the oil and the hands, and applying to a warm body would aid penetration.
- Fatty Acid Saturation
- Yet another influencing factor is the degree of saturation of the vegetable oil; it appears that the greater the unsaturation of an oil, the better the penetration rate. It has benn suggested that increased amounts of short chain and polyunsaturated fatty acid components in vegetable oils favours their percutaneous absorption (Valette & Sobrin 1963) linseed oil crosses the skin more quickly than expected from its viscosity but the skin is resistant to olive oil which is mainly monounsaturated (Scheuplein & Ross 1970) the other hand, there is some evidence that Jojoboa wax, consisting mainly of saturated fatty acids, can permeate the skin. There are photographs have been produced showing the oil in a 'pool' at the base of a hair and moving through the follicule wall into the corneal layer (Anon 1985)
- It would therefore seem until such time as a more clear cut answer is obtained, it is reasonable to continue to say that the molecules of carrier oils are perhaps too large to be absorbed through the skin: Zatz (1990) speaking of a molecule size says that considering the horny layer is a compact membrane and that diffusing molecules must follow a tortuous path through it, it might seem obvious that the diffusion coefficient would be inversely proportional to molecular weight and size; in other words, the bigger the molecule the less the likelihood of it passing the skin barrier.
- Essential oils are completely soluble in all proportions in vegetable oils, and so it must follow that the rate of absorption of an essential oil is governed to a large extent by the characteristics of the vegetable oil employed as the carrier. The fixed oil also influences the amount of essential oil absorbed through the skin because the volatility of the essential oil is decreased and so it remains on the body longer, giving greater opportunity of absorption.
- Reference: Carrier Oils for Aromatherapy and Massage: Len Price with Ian Smith & Shirley Price